The present invention relates to a method for manufacturing a thin-layer component, in particular a thin-layer high-pressure sensor having a substrate on which at least one functional layer to be patterned is to be deposited. The present invention as well as the underlying objective are explained with reference to a thin-layer high-pressure sensor, although they could, in principle, be applied to any thin-layer component.
High-pressure sensors are used in several systems in the motor vehicle industry. Among them are direct fuel injection systems, common rail direct diesel fuel injection systems, electronic stability programs, and electrohydraulic brake systems. High-pressure sensors are also used in the field of automation technology. The function of these sensors is based on using a thin-layer system to convert the mechanical deformation of a stainless steel membrane produced by pressure into an electrical signal. In this context, the deformation and, thus, the output signal are determined by the mechanical characteristics of the membrane and the pressure to be measured.
FIG. 2 shows the design of the thin-layer system of high-pressure sensors typically used today.
An insulating layer 20, usually SiOx, is disposed directly on a steel membrane 10, which is an integrated component of a holding device 100. Four strain gauges 30 (three are shown in FIG. 2) made of NiCr, NiCrSi, or doped poly-Si, for example, are disposed on insulating layer 20. The strain gauges form a Wheatstone""s bridge, which is extremely sensitive with respect to the slightest change in the resistance of the individual strain gauges 30. Strain gauges 30 are contacted via a special contact layer or a corresponding layer system 40 having, for example, the layer sequence, NiCr layer 43/Pd layer 42/Au layer 41 (or Ni). A passivation layer 50, usually an SixNy layer, protects subjacent layer system 40 from external influences.
Due to the measuring bridge""s high degree of sensitivity, it is important that passivation layer 50 completely cover the actual measuring bridge in order to ensure an interference-free operation of the sensor element under the operating conditions of motor vehicles (the sensor element""s contacting surfaces are generally unpassivated).
In representing the thin-layer system, as described above, the thin-layer process typically represents a processing of the individual sensor elements in a larger group, which greatly reduces processing costs. Such a grouping is achieved with the help of a workpiece support in which the individual steel substrates to be coated are placed.
Generally, insulating layer 20 is deposited over the entire surface on the surface to be coated of steel membrane 10. Subsequently, the actual functional layer for strain gauges 30 is deposited over the entire surface. Strain gauges 30 are then produced with the help of a photolithographic patterning step. Subsequently, the contact layer or contact layer system 40, usually also being photolithographically patterned, is deposited. Shadow masking is also used as an alternative to photolithographically patterning contact layer 40. This is often followed by a balancing operation to adjust the desired electrical characteristics (in particular to adjust the symmetry of the bridge). Subsequently, passivation layer 40 is deposited which is also patterned photolithographically or using a shadow mask.
It can be concluded that the currently used manufacturing process necessitates at least one photolithographic patterning step for producing the thin-layer system described above.
For processing a plurality of sensor elements in a workpiece support, such a photolithographic patterning step, which generally includes the individual processes of pre-conditioning, resist coating, pre-baking, exposing, developing, (hard bake) etching, and removing the resist coating, entails numerous difficulties. Maintaining the required geometric tolerances and achieving a sufficient resist coating result represent particular difficulties. Maintaining the geometric tolerances necessitates a very precisely machined and, thus, expensive workpiece support. Moreover, the exposure plane, which is insufficiently defined due to the process tolerances of the individual elements, leads to significant deviations in the attained line widths. Furthermore, in mass production, a significant expenditure is required to prevent the materials from being carried over from one process step to the next.
As a result of all of the stated difficulties, a lithographic patterning step entails significant yield losses.
With respect to known approaches, the manufacturing method according to the present invention has the advantage that the layer construction of the high-pressure sensors used today can be achieved without a single photolithographic patterning step. Thus, the difficulties connected with the photolithographic patterning step are eliminated. Contrary to other proposed thin-layer processes not involving a photolithographic patterning step, the typical layer sequence of the high-pressure sensors currently in use can be maintained. This layer sequence has proven to be particularly successful under the operating conditions in motor vehicles. Thus, complete functionality, including stability throughout the working life, resistance to media, and electromagnetic compatibility, can be ensured despite the alteration in process.
An idea underlying the present invention is that of a thin-layer process for processing high-pressure sensor elements not requiring a photolithographic patterning step for patterning a functional layer. Thus, the functional layer""s pattern, for example strain gauges, is produced using an appropriate laser patterning step.
Substituting the photolithographic patterning step, which is comprised of the above-mentioned individual processes, with a single laser patterning step results in significantly simpler process management. This leads to significantly shorter process times and greatly reduced handling expenditures, which is beneficial for mass production. Instead of a resist coating device, an oven, an exposer, a developer, an etching device, and a resist coating removal device, only an appropriately equipped laser is necessary, which signifies, among other things, a smaller need for clean room area and leads to reduced costs. The increased yield as a result of eliminating the yield-critical process steps, pre-conditioning, resist coating, exposing, developing, and removing the resist coating, also contributes to savings. Due to the smaller demands on the workpiece support, reduced costs can also be expected in this context. As a result of smaller cumulative tolerances, the strain gauges can be more exactly positioned on the sensor element.
Due to the more favorable etching behavior, the NiCrSi, for example, is typically deposited at room temperature in the usual method. After etching using a wet chemical treatment, a thermal treatment is necessary to cause the recrystalization to occur in the desired stage. In the case of laser patterning according to the present invention, the NiCrSi can be directly deposited in the desired stage. The thermal treatment is then unnecessary, i.e., an additional process step involving significant logistical difficulties is no longer needed. Contrary to an etching process using a wet chemical treatment, there is no danger of the substrate, e.g., the steel substrate, being corroded when creating the functional layer""s pattern, for example, the strain gauges, via laser patterning. Selecting the material for the workpiece support is also simplified as a result of the reduced demands with respect to the resistance to media.
According to a preferred further refinement, an excimer laser having an appropriate optical system is used in conjunction with a mask for laser processing to transfer the desired pattern of the functional layer from the mask to the thin-layer component in one exposure step.
According to an additional preferred further refinement, the thin-layer component has a contact layer or a contact-layer system for contacting the patterned functional layer, and using a shadow mask, the contact layer or the contact-layer system is directly deposited with the desired geometric pattern.
According to another preferred further refinement, the thin-layer component has a passivation layer for passivating the patterned and contacted functional layer, and using a shadow mask, the passivation layer is directly deposited with the desired geometric pattern.
When the contact layer and the passivation layer are deposited with the help of shadow masks, absolutely no photolithographic patterning step is required for processing the thin-layer high-pressure sensor elements.
According to an additional preferred embodiment, the thin-layer component is a thin-layer high-pressure sensor where the substrate has a steel membrane and an overlying insulating layer.
According to another preferred further refinement, the functional layer is an NiCrSi layer or a poly-Si layer from which the strain gauges are patterned for a resistance measuring bridge.
According to a further preferred exemplary embodiment, the resistance measuring bridge is adjusted by a laser processing step analogous to the patterning. Thus, two similar process steps can be carried out by one and the same apparatus.